1. Field of the Invention
The invention relates to a matt, biaxially oriented polyester film which comprises at least one matt base layer (B), at least 70% by weight of which is composed of a thermoplastic polyester. The invention further relates to the use of this film and to a process for its production.
2. Description of the Related Art
The film of the invention has characteristic matt surfaces and has good suitability for use as a packaging film or for industrial applications.
The packaging industry has a high requirement for transparent, high-gloss plastic films, e.g. biaxially oriented polypropylene films or biaxially oriented polyester films. In addition, there is increasing requirement for films of this type in which the surfaces are not high-gloss but have a characteristic matt appearance giving, for example, the packaging an appearance which is particularly attractive and therefore effective for promotional purposes.
U.S. Pat. No. 3,154,461 claims a process for producing a biaxially oriented matt-surface film from thermoplastic (e.g. polyethylene terephthalate, polypropylene), in which the film comprises incompressible particles (e.g. calcium carbonate, silicon dioxide) with size of from 0.3 to 20 xcexcm and at a concentration of from 1 to 25%. The application concerned also claims the matt film produced by the process according to the invention. The degree of mattness of the film produced by the process according to the invention is unsatisfactory for many applications.
DE-A 23 53 347 describes a process for producing a milky polyester film having one or more layers, which comprises preparing a mixture made from particles of a linear polyester with from 3 to 27% by weight of a homopolymer or copolymer of ethylene or propylene, extruding the mixture as a film, quenching the film, and orienting the same by orientation in directions running perpendicular to one another, and heat-setting the film. A disadvantage of the process is that it is not possible to reuse the cut material arising during production of the film (essentially a mixture of polyester and ethylene copolymer or propylene copolymer) as regrind without yellowing of the film. This makes the process uneconomic, and the yellowish film produced with regrind was unacceptable to the market. On increasing the concentration of the copolymer in the polyester, the film generally loses its milky character and becomes white with high opacity.
It was therefore an object of the present invention to provide a matt, biaxially oriented polyester film not having the disadvantages of the prior art films mentioned, but having in particular a high degree of mattness, very high ease of production and very good processibility. In addition, care should be taken that the film can also be processed on high-speed machinery. It should moreover be ensured that cut material occurring during film production can be reintroduced into the production process as regrind during production of the film in amounts of up to 70% by weight, based on the total weight of the film, without any significant adverse effect on the physical or optical properties of the film.
According to the invention, the object is achieved by means of a matt, biaxially oriented polyester film having at least one matt base layer (B) which comprises at least 70% by weight of thermoplastic polyester, where the base layer (B) also comprises a pigment system characterized by the following features
a) the median diameter of the pigment grains (d50) is in the range from 2 to 5 xcexcm,
b) the scattering of the distribution of the grain size of the pigment grains, expressed via the SPAN 98, is less than or equal to 1.9, and
the concentration of the pigment grains is in the range from 1.0 to 10% by weight, based on the total weight of the film.